B757-200 DIVERTS WHEN HIGH ENGINE VIBRATION; FOLLOWED BY A LEFT ENGINE GEN DRIVE EICAS WARNING ALERT THE FLT CREW TO A POTENTIAL PHYSICAL BREAKDOWN OF THE LEFT GENERATOR.

Narrative

AFTER LNDG GEAR RETRACTION; THE COPLT AND I FELT A SMALL UNUSUAL AIRFRAME LOW FREQ VIBRATION. IT WAS NOT ENOUGH TO BE CONCERNED ABOUT; BUT UNUSUAL. THE ENG VIBRATION INDICATORS WERE IN A NORMAL RANGE; BUT THE L ENG WAS A LITTLE HIGHER; A MAX OF ABOUT 1.1 ON THE GAUGE. THE R ENG WAS ABOUT .2 CLBING THROUGH FL300; WE GOT A 'L GENERATOR DRIVE' LIGHT ON THE OVERHEAD PANEL; WITH THE ACCOMPANYING EICAS ALERT 'L GENERATOR DRIVE.' THIS MEANS THAT THE INTEGRATED DRIVE FOR THE GENERATOR EITHER HAD LOW OIL PRESSURE OR HIGH TEMP. WE RAN THE GENERATOR DRIVE CHKLIST IN THE FLT MANUAL; DISCONNECTED THE IDG FROM THE ENG ACCESSORY GEAR BOX; AND STARTED THE APU. AS SOON AS WE DISCONNECTED THE GENERATOR; THE AIRFRAME VIBRATION STOPPED. THE VIBRATION INDICATOR WENT FROM 1.1 TO .2 TO MATCH THE OTHER ENG. WE WERE CONCERNED OF OTHER ENG OR ACCESSORY DAMAGE THAT MIGHT HAVE BEEN CAUSED BY A POSSIBLE MECHANICAL FAILURE; WHICH MAY HAVE CAUSED THE VIBRATION. BY THE TIME WE HAD DISCUSSED THE SITUATION; CONSULTED WITH THE ACR MAINT AND DISPATCH; WE WERE NEAR ZZZ ARPT; AND ELECTED TO DIVERT AND LAND THERE. WE DID DECLARE AN EMER IN CASE WE EXPERIENCED AN ENG FAILURE; WE ALSO ADVISED THE FLT ATTENDANTS. THE WX IN ZZZ WAS GOOD; CLR; BUT WITH THE UNUSUAL CIRCUMSTANCE OF THE GENERATOR PROB; AND THE NIGHT FLT; WE FELT IT PRUDENT TO DIVERT AND HAVE MAINT PERFORMED. THERE WAS A NORMAL UNEVENTFUL LNDG. WHEN WE CLRED THE RWY; EVERYTHING WAS NORMAL SO WE ELECTED TO TAXI TO THE GATE. WE ALSO LANDED SLIGHTLY OVER THE MAX LNDG WT OF 198.0; AND LANDED AT 199.9. WE ENDED UP CONTINUING THE FLT WITH ANOTHER PLANE.